This invention relates to a process for converting the aluminum values found in aluminum-containing ores to alumina (Al.sub.2 O.sub.3) which can then be used to produce aluminum metal. More particularly, the invention relates to a process for preparing alumina from aluminum-containing ores such as the aluminum-enriched clays indigenous to the United States. As is known, the major use of alumina is in the commercial production of aluminum metal. There are many known processes for converting the alumina to aluminum, although the well-known electrolysis process is probably the process most widely used commercially.
Commercial aluminum production normally consists of two major operations -- the production of pure alumina and the reduction of this alumina to aluminum metal in the Hall-Heroult cells of the electrolysis process. Although aluminum occurs in many minerals, virtually the only source of alumina which has been used for commercial aluminum production is bauxite, a naturally occurring material comprising mainly hydrated oxides of alumina. Since about 1920, the United States has become progressively more dependent upon foreign sources for its supply of this strategic raw material. By 1970, less than one-eight of the bauxite consumed in the United States was of domestic origin, with most of the bauxite being imported from Jamaica. This situation was aggravated recently by steps taken by the Jamaican government which are expected to drastically raise the cost of imported Jamaican bauxite.
The most widely used method for the production of alumina from bauxite has been the so-called "Bayer Process" which is described in detail in the Bureau of Mines Report of Investigations No. 6730 published in 1960 and entitled "A Cost Estimate Of The Bayer Process For Producing Alumina". In addition to requiring bauxite as its starting material, the Bayer Process requires an undesirable, energy consuming bauxite leaching step involving elevated temperatures and pressures on the order of about 400.degree. F. and 200 p.s.i, respectively.
The domestic reserves of bauxite, mainly in Arkansas deposits, are considered inadequate as a long range supply of aluminum. However, the potential domestic supply in low-grade, non-bauxite alumina deposits such as clays and the like is practically unlimited and, as might be expected, much work has been undertaken to develop commercial processes for extracting alumina from these low-grade domestic materials. Since domestic clays provide an abundant source of aluminum in the form of hydrous aluminum silicates, much of the effort has been focused in this area.
Among the proposed processes for obtaining aluminum from domestic ores such as clays are those in which the clays are leached with various inorganic acids such as HNO.sub.3, HCl, H.sub.2 SO.sub.4 and mixtures of H.sub.2 SO.sub.4 and H.sub.2 SO.sub.3 to extract the aluminum from the clay and ultimately produce alumina which can then be converted to aluminum. In such processes, the clay must ordinarily be calcined or dehydrated to remove water and convert the hydrous aluminum silicates to amorphous oxides of aluminum and silicon, thereby making the aluminum values leachable by the acids. Moreover, certain of the processes require prolonged leaching of the clay at high temperatures and pressures. Various acid-clay leaching processes are described in detail in Bureau of Mines Report of Investigations Nos. 6431 (HNO.sub.3), 6229 (H.sub.2 SO.sub.4), 6133 (HCl) and 7758 (H.sub.2 SO.sub.4 -- H.sub.2 SO.sub.3). None of the acid leaching processes just mentioned compare favorably with the Bayer Process from an economic viewpoint and for this reason have not been widely adopted commercially.
It is also known that aluminum fluoride (AlF.sub.3) for use in the production of aluminum can be prepared by treating clays, which preferably have been calcined, with fluorine containing acids such as HF or H.sub.2 SiF.sub.6. See, for example, U.S. Pat. Nos. 508,796 and 1,403,183 and British Pat. Nos. 15,083 and 643,379. The AlF.sub.3 is recovered from the reaction mixtures as crystals of hydrated AlF.sub.3 which are then heated to remove water and produce the AlF.sub.3 product.
Major drawbacks of prior art processes for producing alumina such as the Bayer process and certain of the acid-clay processes are that they require either an undesirable calcining of the aluminum raw material and/or a high temperature, high-pressure leaching of the aluminum raw material. Other drawbacks involve the need for relatively costly raw materials and, in the case of the Bayer process, the inability to use non-bauxite ore sources. The Bayer process also produces large quantities of useless solid wastes known in the bauxite industry as "red mud". Disposal of this "red mud" presents serious problems.
It is an object of this invention to provide an acid process for preparing alumina from aluminum-containing ores, such as the various aluminum-enriched domestic clays, which offers potential economic advantages over other known processes for producing alumina such as the Bayer Process and the various acid-clay leaching processes.
These and other objects of the invention will be apparent to those skilled in the art from a consideration of the specification and attached drawings taken in their entirety.